Semi-Continious  Non-Methane Organic Carbon Analyzer

ABSTRACT

An instrument to provide on-site analyses utilizing a combination cryotrap and absorbent traps in the extraction of constituents from the sample gas stream consistent with the requirements of 40 CFR Pt. 60 App. A Method 25. 
     The air sample is passed through a cryotrap system which is maintained at the temperature of dry ice ˜72° C.). Heavier molecular weight constituents in the gas sample are condensed on the cold surfaces of the trap. The lighter molecular weight constituents are retained in the absorbent trap, while also acting as a column to separate methane, carbon dioxide, and carbon monoxide from each other and the remain carbon compounds. 
     After separation, all of the sample constituents are oxidized to carbon dioxide and reduced to methane prior to being introduced to a Flame Ionization Detector, which measures the concentration of each against known calibration standards.

BACKGROUND OF THE INVENTION

This invention relates to air pollution analyzers and the like which measure constituents in a sample gas stream. More particularly, the invention relates to a combination cryotrap and adsorbant traps used for extracting constituents from the sample gas stream.

Non-Methane Organic Compounds are defined in and by 40 CFR Appendix A Method 25. The historical data for this methodology therefore must be comparable to future data to ensure all of the data is legally defensible. Thus, any direct measurement systems must be able to be favorably compared with earlier non-directly measured analyses. The only proven method for this comparison is with a physical trapping procedure using cryogenic processes.

In a typical air pollution application, the air sample is passed through a cryotrap which is maintained at the temperature of dry ice. Heavier molecular weight constituents in the gas sample are condensed by the low temperature and solidify on the cold surfaces of the trap while lighter molecular weight constituents are separated or absorbed by the columns After the desired amount of sample gas has passed through the trap, the gas flow path is changed so as to pass a carrier gas through the trap in place of the sample gas. At this time, the traps are heated. The frozen constituents vaporize, the absorbed constituents are desorbed, and both are carried away with the carrier gas to an analyzer or other instrument. The now clean traps are ready for another sampling and flushing cycle.

In the cryotraps commonly used, liquid nitrogen, liquid carbon dioxide, or solid dry ice are chosen for the freezing step. However, the coolant is lost to the atmosphere and a new quantity must be utilized for each operation. For instruments which operate continuously, the cost of these coolants is very high.

It is an object of the present invention to provide a new and improved cryotrap which uses a closed cycle cooler for the freezing, with no coolant being discharged after each sampling cycle.

A further object of the invention is to provide such a cryotrap with a new and improved construction for obtaining increased efficiency in cooling and heating while enabling smaller construction and shorter operating cycles.

Earlier mechanical coolers are bulky, heavy, noisy, and have high mechanical vibration. They have relatively short life cycles or had long cycle times. The present invention is to use a Stirling closed cycle linear cooler which is relatively lightweight and compact. Features of the Stirling cooler include low conducted and radiated emissions, low mechanical vibration, acoustically quiet operation, and improved lifetime and reliability. The energy transfer is much greater and allows for a faster cooling cycle.

Other objects, advantages, features and results will more fully appear in the course of the following description.

SUMMARY OF THE INVENTION

The cryotrap of the invention includes a core having first means for receiving a cooling gas expander, a length of tubing carried on the core defining a gas flow path around the gas expander, where said tubing can be heated via electric resistance. The core preferably has a helical groove around the exterior thereof, with the tubing positioned in the groove. The core may be a cylindrical aluminum rod with a helical groove on the exterior with the tubing wound therein, and an axial opening for the gas expander.

The source of cooling may comprise a Stirling linear drive cooler charged with helium and having a gas compressor connected to one end of the gas expander with the other end of the expander positioned in the core.

Insulation for the cryotrap preferably includes a support for the core having a first central plate and a plurality of second insulator plates, with at least one second plate on each side of the first plate, and with the core positioned in the first plate. Each of the first and second plates is of a low density, closed cell, rigid foam thermal insulation with a plurality of openings therethrough, and metalized plastic film on each side of the plate, and includes crumpled metalized plastic film filling the openings of the second plate. A plurality of third insulator plates may be used with at least one third plate on each side of the first and second plate combination, and a fourth mounting plate on a side of the first, second and third plate combination with the plates joined in a sandwich construction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a Stirling cooler assembly as used in the cryotrap;

FIG. 2 is a view of the core and coil in the assembly of the cryotrap of FIG. 4;

FIG. 3 is an exploded view of the main components of the cryotrap incorporating the presently preferred embodiment of the invention;

FIG. 4 is a view of the assembly of the main components of the crytrap; coil, core, collar and Stirling cooler;

FIG. 5A showing the plumbing pathways through the system using a 4 and 6 port combination;

FIG. 5B is a diagram illustrating the operation of the cryotrap, with a 10 port control valve in the sampling position showing the two flow paths through the valve; and

FIG. 5C is a diagram similar to that of FIG. 5A with a 10 port control valve in the injecting position showing helium carrier flow to the detector; and

FIG. 5D is a diagram similar to that of FIG. 5C showing the flow of helium carrier to the detector with the valve in the backflush position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the cryotrap shown in FIG. 1, a core plate, apertured insulator plates, solid insulator plates, and a mounting plate are held together in a sandwich construction by rods and screws positioned at the corners of the plates. A core is positioned in the core plate and a gas expander is attached to the mounting plate by screws, with the tube of the gas expander extending through openings in the plates and into an axial opening in the core. A thermally conductive compound such as silver-loaded silicone grease may be applied to the end of the expander tube to improve the thermal connection to the core.

The gas expander is part of a refrigeration system which provides for cooling of the core. In the preferred embodiment illustrated, a Stirling linear cooler is utilized, having a compressor connected to the gas expander. The Stirling linear cooler may be of conventional design, providing a closed cycle with helium being compressed at the compressor and with pressure pulses transferred through the helium to the expander. Cooling is obtained by cyclic out-of-phase motion of a compression piston and a displacer-regenerator located in the expander assembly.

The compressor is operated for a time prior to the introduction of the sample to allow the core and tubing to reach the desired operating temperature. The compressor continues to operate during the time that the sample is passed through the trap tubing. The compressor is then turned off during the heating mode.

The core preferably is a aluminum disc with a helical groove on the exterior. A length of tubing, preferably of stainless steel, is wound on the core in the helical groove. Temperature sensor may be positioned in openings in the core, preferably between the gas expander opening and the helical groove. The temperature sensor is connected to a control circuit by wires.

The stainless steel tubing of the trap is heated via the electrical resistance of a DC current passing from one end through the other.

During the cooling modes, the compressor control electronics hold the core temperature to a selectable preset temperature as sensed by the temperature sensor. The heater temperature controller also provides a display of the core temperature. Cooling and heating modes are controlled by the cryotrap sequence control electronics, with power from a DC power supply.

Desirably, the core plate and insulator plates are formed of a low density, closed cell, rigid foam for thermal insulation, typically a polymethacrylimide foam. The individual plates preferably are about ½″ thick and about 4″ square. A layer of moralized plastic film, typically aluminized Mylar film, is positioned on each side of each plate, typically about 0.0005″ thick.

A plurality of openings are provided in each of the core plate and insulator plates, typically about ½″ in diameter. Prior to applying the surface layers, the openings preferably are filled with crumpled metalized plastic film, typically aluminized Mylar film. Also, preferably the openings in adjacent plates are misaligned.

In operation, the core is cooled by the gas expander rod in the opening of the core. Substantial insulation is provided around the core and the gas expander tube so there is minimal heating from the surrounding atmosphere. At the same time, heat is periodically applied to the tubing for thawing the frozen gas constituents. It has been found that the insulation construction using the foam plates, with the crumpled metalized film in the openings achieves an excellent balance between heating and cooling, permitting freezing of substantially all constituents in the gas while at the same time requiring a minimum of cooling energy and permitting a rapid cycle time.

It is desirable to cool the core quickly. Rapid cooling minimizes wear on the Stirling cooler and enables the cryotrap to complete a cooling/heating cycle quickly. To cool the core quickly it is necessary to minimize the amount of material that must be cooled. This includes the insulating material around the core, therefore insulating material is removed resulting in a plurality of openings in each of the core plates and insulator plates. The openings are typically ½″ in diameter leaving a “web” of foam insulation to support the core and providing a longer path through the foam from the core to the outside. To prevent air currents from circulating in these openings, the openings are filled with crumpled metalized plastic film, typically aluminized Mylar film. Metalized plastic film is also bonded to each side of the insulating plate. The metalized plastic film also serves to reflect external radiant energy away from the core.

The cryotrap is utilized with a ten port valve which is movable between a sampling position shown in FIG. 5B and a flushing position shown in FIGS. 5C and 5D. A source of sample gas is connected to the valve through a line, and a vent for the sample gas is connected to the valve through another line. The carrier gas from an analyzer such as a gas chromatograph is connected to the valve by an incoming line and an outgoing line. The tubing is connected to the valve.

During the sampling mode, the sample gas flows through the line, the valve, through the tubing, and back through the valve to the vent line, with the carrier gas flowing directly to and from the valve to the detector.

With the valve turned to the flushing mode, the sample gas flows directly into and out of the valve, with the carrier gas flowing through the line to the valve, through the tubing and back to the analyzer through the valve and the line.

The operation of the valve may be automatic, operating on a predetermined cycle or may be manually operated as desired.

If a faster analytical cycle is desired, a second cryotrap assembly may be added along with a 6 port valve to switch between traps. In such a case one trap would be cooling while the other is used for the sampling and analysis.

REFERENCES

Mitra S., Yun C. “Continuous gas chromatographic monitoring of low concentration sample streams using an on-line microtrap” J. Chromatogr. A 648, 415-421 (1993)

1. Feng C. H., Mitra S. “Two-stage microtrap as an injection device for continuous on-line gas chromatographic monitoring” J. Chromatogr. A805, 169-176 (1998)

Mitra S. “Analytical apparatus and instrumentation for on-line measurement of volatile organic compounds in fluids” U.S. Pat. No. 6,112,602 Sep. 5, (2000)

40 CFR 60. Appendix A to Part 60 Method 25—Determination of total gaseous nonmethane organic emissions as carbon

US PATENT REFERENCES

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1. A device for the semi-continuous measurement of Non-Methane Organic Carbons (NMOCs) and including Volatile Organic Compounds (VOCs) in a sample stream without chromatographic separation of the NMOCs, comprising: a sampling and concentration system connecting said sample stream to a detector for said NMOCs and VOCs; said sampling and concentration system comprising one or more multi-port valves as a means to connect said sample stream to a cryotrap and an adsorbent trap; said cryotrap being capable of cooling and rapid heating which cryogenically traps the NMOCs and VOCs in said sample stream; said adsorbant trap being capable of rapid heating and cooling, and containing adsorbent composition(s) which adsorbs the NMOCs and VOCs in said sample stream; said one or more multi-port valves connecting to said traps which allow the venting out of CO, CO₂, CH₄, and other gases in said sample stream so as to concentrate said NMOCs and/or said VOCs in said trap; desorption means associated with said traps whereby said trapped/adsorbed NMOCs and/or VOCs are rapidly desorbed therefrom and injected into said detector; carrier means connecting said sample stream to said traps and said detector; whereby the one or more multi-port valves operate simultaneously so as to pass said carrier through the traps into said detector so as to carry said desorbed NMOCs and/or VOCs from said traps into said detector as a concentrated pulse; wherein said detector is selected from the group consisting of a flame ionization detector (FID), an infrared spectrophotometer, or other suitable detectors or sensors for said NMOCs and VOCs.
 2. A device according to claim 1 wherein the multi-port valve comprises a gas sampling valve which makes an injection onto the traps, which retains the NMOC and vents said CO, CO₂, CH₄, and other gases, prior to desorption of the NMOC into said detector.
 3. A device according to claim 1 wherein the multi-port valve comprises a ten port or a six port and four port valve operating simultaneously with the trap connected there between.
 4. A device according to claim I wherein the multi-port valve passes the sample through the trap while venting said CO, CO₂, CH₄, and other gases, and further, the multi-port valve is switched such that the flow direction of carrier through the trap is reverse of that of the sample to allow backflushing of the trap during desorption into said detector
 5. A device according to claim 1 where the device is operated continuously and repetitively, so as to provide near real-time monitoring of said sample.
 6. A device according to claim 1 wherein the said detector comprises an oxidation means to convert the organic compounds to carbon dioxide which is then further converted to methane using reduction and connected in series with the oxidation, and measuring the thus-produced methane using an FID, or other suitable detectors or sensors for said NMOCs and VOCs.
 7. A device according to claim 6 wherein the oxidation and reduction are catalytic and/or thermal.
 8. A device according to claim 7 wherein the carbon monoxide, carbon dioxide, and methane may also be measured by an FID or other suitable detector,
 9. A device according to claim 1 containing a cryotrap, the combination of: a core having first means for receiving a cooling gas expander; a length of tubing carried on said core defining a gas flow path around said gas expander on said core; which said tubing is connected to a transformer for resistive heating. said core having a groove around the exterior thereof, with said length of tubing positioned in said groove. said core and tubing assembly being contained within an aluminum housing which is clamped onto the expander housing.
 10. A cryotrap according to claim 9 wherein said core is an aluminum disc with a groove around the exterior with said tubing wound therein, with an axial opening for said gas expander.
 11. A cryotrap according to claim 10 including a second opening in said core between said axial opening and said groove for receiving a temperature sensor.
 12. A cryotrap according to claim 11 including a Stirling linear drive cooler charged with helium and having a gas compressor connected to one end of said gas expander with the other end of said expander positioned in said core.
 13. In a cryotrap for an NMOC analyzer, the combination of: a core having first means for receiving a cooling gas expander, a length of tubing carried on said core defining a gas flow path around said gas expander, and a support for said core having a first central plate and a plurality of second insulator plates, with at least one second plate on each side of said first plate, and with said core positioned in said first plate.
 14. A cryotrap according to claim 13 including a Stirling linear drive cooler charged with helium and having a gas compressor connected to one end of said gas expander with the other end of said expander positioned in said axial opening of said core.
 15. A method for the continuous measurement of Non-Methane Organic Compounds (NMOCs) and/or Volatile Organic Compounds (VOCs) in a sample stream without chromatographic separation, comprising: providing a sampling and concentration system connecting said sample stream to a detector for said NMOCs and/or VOCs; said sampling and concentration system comprising one or more multi-port valve connecting said sample stream to a cryotrap and an adsorbent trap; said adsorbent trap being capable of rapid heating and cooling, and containing at least one adsorbent composition which adsorbs the NMOCs and/or VOCs in said sample stream; said multi-port valve connected to said trap which allows the venting out of CO, CO₂, CH₄, and other gases in said sample stream so as to concentrate said NMOCs and/or VOCs in said trap; desorption means associated with said adsorbent trap whereby said adsorbed NMOCs and/or VOCs are rapidly desorbed therefrom and injected into said detector; carrier means connecting said sample stream to said cryotrap, adsorbent trap, and said detector; whereby the one or more multi-port valves operate simultaneously so as to pass said carrier through the adsorbent trap into said detector so as to carry said desorbed NMOCs and/or VOCs from said adsorbent trap into said detector as a concentrated pulse; adsorbing the NMOCs and/or VOCs in the sampling and concentration system while venting out the CO, CO₂, CH₄, and other gases in said sample stream; desorbing the NMOCs and/or VOCs from said adsorbent trap; and injecting the desorbed NMOCs and/or VOCs into said detector; wherein said detector is selected from the group consisting of a flame ionization detector, an infrared spectrophotometer, and other suitable detectors or sensors for said NMOCs and VOCs.
 16. A method according to claim 15 wherein the multi-port valve comprises a gas sampling valve which makes an injection onto the trap which retains the NMOCs and/or VOCs and vents said CO, CO₂, CH₄, and other gases, prior to desorption of the NMOCs and/or VOCs into said detector.
 17. A method according to claim 16 wherein the multi-port valve comprises a ten port or a six port and four port valve operating simultaneously with the trap connected there between.
 18. A method according to claim 17 wherein the multi-port valve passes the sample through the trap while venting said CO, CO₂, CH,₄, and other gases, and further, the multi-port valves are switched such that the flow direction of carrier through the traps is reverse of that of the sample to allow backflushing of the traps during desorption into said detector.
 19. A method according to claim 18 for measuring organics wherein the stripped organics are measured using a detector that is selected from a flame ionization detector, a GC, a mass spectrometer, an infrared spectrophotometer, and any other suitable detector or sensor.
 20. A method according to claim 19 where the process is repeated continuously and repetitively, so as to provide near real-time monitoring of said sample.
 21. A method according to claim 20 wherein the said detector comprises an oxidation means to convert organic to carbon dioxide which is then measured, or further converted to methane using reduction connected in series with the oxidation, and measuring the thus-produced methane using FID.
 22. A method according to claim 21 wherein the oxidation and reduction are catalytic and/or thermal.
 23. A method according to claim 22 wherein the carbon monoxide, carbon dioxide, and methane may also be measured by an FID or other suitable detector. 